1. Field of the Invention
The present invention generally relates to the field of fabricating mass products, such as integrated circuits, wherein a plurality of manufacturing and metrology steps are carried out by corresponding process tools and metrology tools. More particularly, the present invention relates to the substrate handling in a production line including at least one cluster tool having a plurality of process modules to increase throughput.
2. Description of the Related Art
In manufacturing mass products, a plurality of process steps are typically required wherein control measurements have to be performed on a regular basis to ensure product quality. A typical example for a technologically sophisticated mass production is the fabrication of integrated circuits in which a huge number of quite complex processes are carried out that may be preceded or followed by corresponding metrology steps to precisely monitor the quality of respective process sequences. Process quality and, thus, product quality, however, is only one issue that has to be taken into account by semiconductor manufacturers. A further important criterion for the economic success of a product is the overall throughput achieved in the semiconductor production line. For this reason, so-called cluster tools are increasingly employed which may have a plurality of substantially identical process modules to process a plurality of substrates in a substantially parallel manner.
FIG. 1 shows a simplified schematic view of a typical cluster tool that may be used for etching semiconductor substrates. In FIG. 1, a cluster tool 100 comprises a substrate handling platform 101 which is also referred to as a mainframe. Attached to the mainframe 101 are a substrate input port 102 and a substrate output port 103. Typically, the ports 102, 103 are configured to receive a predefined number of substrates provided in a corresponding substrate carrier (not shown). A typical number of substrates within one carrier, also referred to as a lot, is 25. The cluster tool 100 further comprises a plurality of process modules 104, which in the present example are substantially identical etch chambers indicated by a, b, c and d. It should be noted, however, that the cluster tool 100 may comprise additional process modules that may not be identical to the modules 104. Such additional process modules may, for example, represent a clean or rinse station provided upstream and/or downstream of the process module 104.
The operation of the cluster tool 100 will be described with reference to the formation of contact vias, which in sophisticated integrated circuits requires the etching of high aspect ratio openings having a diameter of approximately 0.1 μm or even less. A typical process flow for etching the contact vias with metrology steps associated therewith will now be described with reference to FIG. 2.
In FIG. 2, a process sequence 200 is illustrated in a simplified form to produce vias having a specified critical design dimension (CD), i.e., having a specified diameter. For convenience, only the relevant steps of the process sequence 200 are illustrated in FIG. 2. In step 201, a mask is created on the substrates by sophisticated photolithography techniques so that the vias may be etched into the underlying material layer or layers using this mask. In step 202, the critical dimension, i.e., the diameter of the openings of the mask, are measured, for example, by employing metrology tools, e.g., via optical measurement instruments such as scatterometers and the like. Thereafter, in step 203, the vias are formed by etching the substrates. To this end, a predefined number of substrates, for example, a lot, is provided to the input port 102 and is distributed among the plurality of process modules 104. Although the modules a, b, c and d are substantially identical, the process parameters and, thus, the process conditions in each of the modules may more or less vary, which may result in a different product quality and in slightly different process times. For an optimized tool utilization, the cluster tool 100 is typically configured so as to obtain a maximum number of processed substrates per time, requiring the decision on which substrate is delivered to which process module to be made on the basis of the current status of the individual process modules 104. Thus, it may hardly be predetermined which wafer is loaded in which process module without significantly lowering the throughput of the cluster tool 100.
After completion of the via etch in step 204, a so-called defect scan may be performed to monitor the level of any defects that may be caused by by-products generated in one of the process modules 104. In step 205, the critical dimension of the actual via is measured, for example, by a scatterometer and the like, thereby detecting any drifts of parameters during the via etch process in step 203, which may be caused, for example, by a pressure variation or the like. An increased measurement sensitivity to a parameter drift during the process sequence 200 may be obtained by determining the difference of the critical dimension of the via mask obtained in step 202 and the critical dimension of the actual via obtained in step 205.
As previously noted, the process modules a, b, c and d may be substantially identical; however, these process modules are operated largely independently so that variations in the process sequence 200 may be caused by some of the process modules 104, whereas others are still operating within the tightly set process conditions. An illustrative example for an operation sequence of two substrate lots, each including 25 substrates, may be as follows. For a lot I, the substrates may be processed by the modules a, b, c and d according to Table 1.
TABLE 1Substrate12345678910111213141516171819202122232425ModuleABCDABCDABCDABCDABCDABCDA
A lot II may be processed by the modules a, b, c and d according to Table 2, wherein, as an example, module b may have been taken offline after etching substrate 6 owing to, e.g., a measurement event indicating a high defect level generated in module b.
TABLE 2Substrate12345678910111213141516171819202122232425ModuleABCDABCACDACDACDACDACDACD
For economical reasons, not all processed substrates are measured in the process monitor measurement steps 202, 204 and 205. Instead, a so-called lot sampling rate is defined which indicates a certain fraction of all lots to be subjected to the metrology operations. Typically, only a few substrates from each lot selected for metrology are actually measured and may have been picked randomly or according to frequently used selection schemes such as: measuring substrates from defined slots of the substrate carrier, for example, slots 5, 10 and 15; measuring the first and the last substrate of the lot; or measuring always the same substrate throughout the whole process sequence 200, i e., in step 202 and step 204 the same substrate or substrates are measured.
These schemes for selecting the substrate to be measured may not allow the operation conditions to be monitored in each of the modules a, b, c and d on a regular basis as the designation of a process module to a substrate depends on the cluster tool 100. This means that recognition of a parameter drift of a process module beyond the tightly set process conditions may be unduly delayed, thereby significantly adversely affecting the yield of the process sequence 200. This problem will be illustrated by the following example. It is assumed that the lot sampling is set to 25%, i.e., every fourth lot receives a CD measurement 202 and 205 and a defect scan 204.
It is further assumed that the substrates in slots 5, 10 and 15 are to be selected for these measurements. A plurality of lots, indicated by L1, L2, L3 . . . may be processed in the cluster tool 200, wherein the wafers of lot L1 to be measured have been processed by process modules a, b and d. Upon processing lot L2, process module d may start contaminating the substrates with particles due to any by-products produced in this chamber. Hence, as the lot sampling rate is set to 25%, L2, L3 and L4 are not measured, and only L5 is measured again. The substrates measured in lot L5 placed to slots 5, 10 and 15 may represent process modules c, a and b. Thus, the increased defect level generated by process module d is not detected. Subsequently, lots L6, L7 and L8 are processed but not measured. The three substrates from lot L9 may then represent the modules b, c and a, so that again the increased defect level created by module d is not detected. The next lot to be measured is L13, while lot L14 is already etched by the cluster tool 100, wherein finally one of the substrates processed in process module d may be in one of slots 5, 10 and 15 and will be subjected to measurement. The high defect level is now detected and module d may be taken offline.
In this illustrative example, 13 lots have been processed after the first occurrence of a non-tolerable defect level in module d, until finally module d has been identified as producing substrates not fulfilling the specifications compared to only 31 substrates (5 lots) that would have ideally been affected under the given lot sampling rate. It should be noted that the illustrative example described above represents a reasonable average value for the “picking” probability. In other cases the number of “out-of-specification” substrates may be higher or lower, but in the long run significantly higher than the number of defect substrates generated under ideal measurement conditions. Since typically the defect lot sampling rate is selected to provide for a reasonable compromise between economic concerns and an acceptable risk for the occurrence of defect substrates, in using cluster tools within a manufacturing sequence, the lot sampling rate has to be sufficiently increased, thereby reducing throughput.
In view of the above-described problems, it is, therefore, highly desirable to provide methods and systems that allow an efficient utilization of cluster tools without unduly increasing the number of substrates that do not meet the process specification.